In the construction of heating and cooling systems and the conduits required for these purposes it is necessary to have fully insulated tubes for the heating or cooling medium available in order to avoid high labor costs incurred by manual insulation of the tubes. This primarily applies to tubes having a small external diameter since manual insulation appears reasonable only for tubes having an external diameter of at least 20 to 25 mm or more.
On principle, the problem of continuous tube insulation for the manufacture of a pre-insulated tube has already been solved in that, for example, an endless strip of soft pre-fabricated foam is wound about the tube and is welded together along the abutting longitudinal sides. However, this process has several disadvantages. In the first place, difficulties arise in this process if the tube is to be sheathed in a foam jacket of larger diameter. Moreover, this process does not warrant firm adhesion of the foam jacket to the tube, so that a space remains between the tube itself and the thermal insulation sheath, which not only detracts from the thermal insulation properties but also facilitates the formation of perspiration water with the ensuring corrosion problems. Moreover, the process still requires two different processing steps, namely the production of the foam strip and the application of said foam strip on the tube.
However, also pre-insulated tubing is available on the market which is provided from the first with a foam jacket of thermoplastic material closed on all sides. The technology used for this purpose is based on coextrusion of a foamable thermoplastic material onto a metal tube by means of an extruder provided with an extruder head. With this process, however, no low density foam jacket is obtained with insulating properties meeting the most rigid standards. Since upon the emergence of molten material from the extruder die not only the external diameter but also the internal diameter of the extrudate increases, it is extraordinarily difficult to attain good adhesion of the foam jacket to the tube. In order to circumvent this problem in said known process it is necessary to limit the radial expansion of the foam jacket in the external direction with the result that the synthetic resin foam, which cannot freely expand, cannot reach the desired low density.
A pre-insulated tube is to have substantially the following properties:
(1) It should warrant excellent mechanical protection of the tube strand, be it of metal or plastic material, against shock, abrasion and crushing;
(2) an excellent thermal insulation coefficient meeting the most rigid standards;
(3) perfect adhesion of the foam jacket to the tube in order to avoid any free space between the foam jacket and the tube itself, especially in view of the formation of perspiration water and the ensuing corrosion problems;
(4) virtually complete imperviousness of the thermally insulating jacket to water vapor.
(5) The foam jacket should be soft and flexible, especially for metal tubes made of copper, in order to permit unproblematic laying and winding on reels.